The present invention relates to a junction inspection method for electronic parts, particularly to an inspection methods for electronic circuit boards in which the soldering condition of the soldered portions is inspected through its temperature distribution.
Junctions of electronic parts such as the soldered portions of electronic circuit boards often have defects, and several kinds of junction inspection methods have already been proposed. For example, whether soldering is nondefective or not is judged by visual inspection to examine if solder is distributed in adequate shape to the portion to be soldered. Japanese patent laid-open No. 59-202048 entitled "ELECTRONIC PARTS-CONNECTED STATE INSPECTION METHOD" discloses a method involving measuring charges in temperature increases per unit time due to infrared rays radiated from conductive patterns when an energy beam is irradiated on lead wires of electronic parts connected to conductive patterns on a circuit board and determining that the soldering is defective if the measured data are large and is nondefective if the data are small.
Furthermore, Japanese patent laid-open No. 61-241641 entitled "SOLDERING INSPECTION APPARATUS" discloses the apparatus for inspecting the soldered portions of flat package type integrated circuits to determine if the soldered portions are nondefective or not by measuring the temperature increase per unit time of the soldered portions by heating the terminals of the integrated circuits and comparing the temperature increase with that premeasured for nondefective soldered portions.
Both Japanese patent laid-open Nos. 59-202048 and 61-241641 disclose a method to determine if soldered portions are nondefective or not by measuring the amount of solder attached to the junctions of electronic parts being judged for nondefectiveness of the soldered portions, on the basis of the temperature increase-change per unit time of the soldered portions. It is very hard to determine if the soldering is nondetective or not by visually inspecting the soldering state and this often causes inaccurate judgements.
Microminiaturization and high precision are being required for electronic circuit boards in recent years and the conventional methods of inspection based on the temperature increase-change per unit time on a point of a soldered portion or inspection of the amount of solder only cannot provide sufficient inspection in that it is difficult to detect defects such as blow holes or pin holes formed in soldered portions or bridges formed over adjoining junctions, for example.